The technique of digital radio communication has been developed, and sophisticated modulating methods are adopted in the radio communication. For instance, the digital TV broadcasting adopts the following system in order to expand a service area: as shown in FIG. 1, a signal transmitted from transmitter station 10 is received by receiver stations 12, 13 via relay station 11. Relay station 11 is formed of an amplifier and an antenna in order to receive an RF signal from transmitter station 10 and amplify it before redelivering the RF signal at the same or different frequency. Such relay station 11 is strongly required to be highly efficient between receiving and redelivering the signal in addition to maintaining of linearity and frequency characteristics. For improving the linearity, distortion compensation techniques are available, and a pre-distortion method among others draws attention due to its high performance.
However, a conventional pre-distortion technique tends to have a greater delay because it uses a base-band signal as a reference signal, so that this technique does not suit for relay devices. A signal in IF band can be used as a reference signal in order to reduce the delay; however, this case does not satisfy the performance required to the relay device discussed above.